The study of the organization of immunoglobulin and T cell receptor genes in leukaemias and lymphomas depends on our ability to obtain high molecular weight DNA. We have compared two different methods of DNA extraction: one being enzymatic, using the enzyme proteinase K, and the other chemical, using urea as a substance purifying DNA. Since the amount and purity of the DNA obtained are similar with each of the two methods of DNA extraction, both are valid to make such a genetic analysis.